Otolith shape analysis has been one of the most used approaches to study population structure in the past decades. Currently, two sets of shape descriptors are used to perform otolith shape analysis, namely, Elliptic Fourier descriptor (EFd), which focuses on the overall otolith shape differences, and Discrete Wavelet descriptor (DWd), which is sensible to local differences along the otolith contour. Here, the authors conducted a comparative analysis of the performance of both the descriptors in reconstructing the population structure and connectivity patterns in a small pelagic fish species with a wide geographical distribution and fast growth rate, the European sardine Sardina pilchardus (Walbaum, 1792), for the first time. A combination of each otolith shape descriptor and shape indices was explored using multivariate statistical methods. The two otolith shape descriptors showed similar, although limited, overall classification success associated with the population dynamic characteristics of the species. Both descriptors point to migration among adjacent areas, such as northern Atlantic locations, eastern Mediterranean and even across well-defined physical obstacles, such as the Strait of Gibraltar, among Atlantic and western Mediterranean locations. Both descriptors supported the division of the populations of Mediterranean waters into three main groups but slightly differed in the group limits of the Atlantic waters. A comparison of the present results with those from previous otolith shape analysis studies using EFd on a decadal time scale revealed differences in the population structure and connectivity patterns compared to the earlier period. These differences not only may be attributed to changes in environmental variables leading to changes in population dynamics but can also be the result of the sardine biomass sharp decrease that occurred in the past decade.

Fish otolith shapes record ecological information of fish and are an important tool in taxonomic, phylogenetic, and dietary studies. Shape and ontogenetic variations in the otoliths of ocellated icefish (Chionodraco rastrospinosus) in the Bransfield Strait, northern Antarctic Peninsula, were analyzed. Ontogenetic changes in otolith morphology were evident. The size of the otoliths mainly grew along the antero-posterior axis. The rostrum and pararostrum developed more than the antirostrum and postrostrum. Otolith variation occurred mainly in the dorsal side compared to ventral side. A row of small holes in the central region of the medial side and an irregular protruded structure on both sides of them, were specific diagnostic characteristics of C. rastrospinosus otoliths compared to other species of Chionodraco. Based on hierarchical clustering analysis, four types of otolith shapes are clearly distinguished, corresponding to larval, juvenile, young, and adult stages of C. rastrospinosus. This work contributes to the understanding of the ecology of commercially important benthic fishes and provides key information for ecomorphology study and fishery management of this species. We suggest the effect of ontogeny needs to be considered when otolith shape data are used to study population structure and life history of this species.

Knowledge on population units is critical for understanding population dynamics and providing an adequate fisheries management. Despite its vast distribution and commercial importance, there is no information on black seabream population structure. Different approaches to otolith analysis were used to elucidate on the stock structure of this species. Otoliths from seven areas along the eastern Atlantic: English Channel, Bay of Biscay, Galicia (Spain), Peniche (west coast of Portugal), Algarve (south coast of Portugal), the Canary Islands and Angola were compared through elliptical Fourier analysis and oxygen and carbon stable isotopes ratio. Otolith shape analysis data achieved 80% of correct assigned specimens in discriminant analysis while poor correct assignment levels were achieved with the stable isotopes ratio. However, the PERMANOVA conducted on δ13C and δ18O values revealed significant differences between all areas, with exception of the two Portuguese areas. The results of this study indicate that black seabream spends most of its life in the same area and that the studied populations should probably be considered as different stock units. However, some mixture was detected in the European region pointing for the usefulness of life history parameters and genetic studies to enlighten the stock structure of the species.

Length-mass relationships and linear regressions are given for otolith size (length and height) and standard length (LS ) of certain mesopelagic fishes (Myctophidae, Paralepididae, Phosichthyidae and Stomiidae) living in the central Mediterranean Sea. The length-mass relationship showed isometric growth in six species, whereas linear regressions of LS and otolith size fit the data well for all species. These equations represent a useful tool for dietary studies on Mediterranean marine predators.

Otolith microstructure of reared and wild cresthead flounder Pseudopleuronectes schrenki larvae and juveniles was used to investigate the daily periodicity of ring formation, morphological change and unique otolith structure related to important life events. By comparing microstructural features of P. schrenki with those reported for other flatfish species, it was shown that there may be microstructural features that are common to all flatfishes. In the sagittae and lapillus, a check (a distinct ring) was formed in the centre of otoliths at c. 6 days post hatching, and the daily formation of rings observed outside the check was confirmed. During metamorphosis, accessory primordia (AP) of otolith growth were formed on the outer edge of the sagittae, and the shape of the sagittae became more complex. No AP was formed on the lapilli, however, and otolith rings were concentrically formed throughout the larval and juvenile (≤51·6 mm standard length, LS ) stages. It is proposed, therefore, that lapilli are more appropriate than sagittae for analysis throughout the larval and juvenile (≤51·6 mm LS ) stages. During metamorphosis, unique rings that are relatively wide and show weak contrast are formed on lapilli (metamorphosing zone, MZ). Hence, the duration of metamorphosis, larval duration and the days of juvenile life can be estimated by the number of rings within the MZ, using rings from the check to outermost ring of the MZ, and that of rings formed outside MZ, respectively. The formation of AP on sagittae as well as the absence of AP, bilateral asymmetry and the formation of a unique structure during metamorphosis on lapilli have also been reported for other flatfishes.